1. Field of the Invention
The present invention pertains generally to inkjet printers and the like and more particularly to printhead data and control circuitry for wide-array printers.
2. Description of the Related Art
Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the recording medium as the printhead is moved relative to the medium. The medium is typically shifted each time the printhead has moved across the medium. The thermal inkjet printer is fast and quiet, as only the ink strikes the recording medium. These printers produce high quality printing and can be made both compact and affordable.
In one prior art design, the inkjet printhead generally includes: (1) ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice; (2) a metal nozzle member in which the orifices are formed in the required pattern; and (3) a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber.
To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a-vaporization chamber, causing a droplet of ink to be ejected through an associated orifice onto the recording medium.
One prior art print cartridge is disclosed in U.S. Pat. No. 4,500,895 to Buck et al., entitled "Disposable Inkjet Head, " issued Feb. 19, 1985 and assigned to the present assignee.
In a thermal inkjet printhead incorporating these types of discrete printheads, the thin film heaters are selectively energized while a mechanism transports the printhead across a recording medium, typically a sheet of paper. The recording medium is incrementally moved perpendicular to the travel path of the printhead so as to enable printing at virtually any location on the recording medium.
In order to selectively energize the individual thin film heaters, a printhead element is associated with each heater. The printhead element typically consists of a diode or a transistor that can be selectively enabled. Typically, a select line is associated with each printhead element which enables the printhead element when a select signal is received on the select line. In order to minimize the number of select lines, the printhead elements can be arranged in a matrix configuration. In the matrix configuration, the select lines are commonly connected to a plurality of printhead elements, each element having a separate supply line. Thus, a printhead element is selectively enabled by generating a select signal on the appropriate select line and enabling a supply signal on the appropriate supply line. After the printhead element is enabled, a current is produced therein which is passed through the corresponding thin film heater. A typical example of a matrix-type inkjet printer driver is shown in European Patent Application No. 441,635 by Matsumoto et al.
To increase the speed of printing per line on a medium and to reduce the mechanical complexity of a printer, it is known to mount separate printheads side by side to form a fixed array of printheads extending across an entire width of a medium. Selected printing elements across the array of discrete printheads are energized simultaneously to print an entire line of dots onto the medium. After the line is printed, the medium is incrementally shifted perpendicular to the array of printheads, and the printing process is repeated.
Drawbacks to this construction of an array of discrete printheads include increased electrical complexity, difficulty in precisely aligning the printheads with one another, and increasing cost in the providing the plurality of printheads.
As is apparent, with resolutions of inkjet printers becoming greater than 300 dots per inch ("dpi"), alignment of the orifices between discrete inkjet printheads across an array of eight inches or more requires extremely precise positioning to achieve satisfactory spacing between printed dots on a medium. This alignment must be maintained throughout the useful life of the product and under different conditions of duty cycle, temperature, shock, and vibration.
Furthermore, as the resolution increases, the amount of data required to selectively energize the individual thin film heaters grows geometrically. For example, a 300 dpi by 300 dpi printer produces 9000 dots per square inch. In contrast, a 600 dpi by 600 dpi printer produces 36,000 dots per square inch. Thus, doubling the horizontal and vertical resolution quadruples the dot density. In order to selectively energize the corresponding thin film heaters, the rate of the data needed to select the desired thin film heaters must increase in geometric proportion to the increase in the dot density.
In addition, as the resolution increases so does the number of thin film heaters. The size of the integrated driver circuits increases correspondingly. The yield of the silicon devices, however, is inversely proportional to the size of the die. Therefore, as the size of the printhead elements to accommodate the increased number of thin film heaters, the yield of the silicon dies reduces.
Thus, what is needed is a driver design that is flexible in order to accommodate the optimal number of thin film heaters to maximize the yield. Also, what is needed is an improved wide printhead structure which requires a reduced data rate and where precise alignment of the orifices across the printhead may be accomplished simply and precisely maintained over the life or the product and over a wide range of operating conditions.